Conventionally, as described in Patent Document 1, a forming method is known, by which a first tapered portion is formed by contracting a diameter of an original tube so that the tube diameter is gradually decreased in the direction of an opening end of the original tube, a second tapered portion is formed so as to continue to the first tapered portion, the second tapered portion having a diameter gradually decreasing at a rate different from the rate at which the tube diameter of the first tapered portion gradually decreases, and the first tapered portion and the second tapered portion are folded and bent inside the original tube to be formed into double-layered structure. In this case, forming one end of the original tube into double-layered structure allows a portion having the double-layered structure to exhibit increased strength and thickness. Accordingly, even with a relatively thin original tube, it is possible to form threaded holes or the like, to weld or the like, at the portion having the double-layered structure.